The Corner effect on inclined near surface cracks.

The ultrasonic detection of cracks using the corner effect is strongly influenced by the inclination of those cracks. The mechanism of the well known corner effect, that means the double reflection of an ultrasonic wave at a corner, a notch or a crack requires two mirroring surfaces tilted against each other with an angle of 90°. As usual for many ultrasonic defect detection tasks the use of this mechanism for crack detection is limited in case of inclined cracks. Whereas this limitation can easely be estimated for planar reflectors far from the surface, the prediction of the inclination dependency of the corner effect is heavily influenced by mode conversions.

In order to define a reference between the sensitivity settings based on notches or side drilled holes and the detectability for near surface cracks under realistic conditions, a number of typical reference reflectors and typical deviations from their ideal orientation have been investigated. The interpretation of the complex reflection on inclined notches has been supported by a theoretical model. This model uses an analytical description of the probe sound field for the reception and the transmission mode, taking into account the elasto-dynamic character of the wave propagation in solid media by simple point directivity functions for longitudinal and shear waves. The pulse shape isdescribed by a quadrupol n model of a piezoelectric transducer element on a perspex wedge. The medium is assumed to be homogeneous and isotropic. The interaction at a crack like defect is taking into account at least 9 possible different sound pathes, among them the classical corner effect, diffraction modes, shear /longitudinal-wave mode conversions direct shear wave reflection e.t.c. The mode conversion at the different surfaces (omposite surface of the plate and crack surface) are taken into account by special point directivity patters for longitudinal and shear waves.

The A-scans are gained by a Fourier transformation. The program works on usual PC's. The present contribution compares different notches with inclinations from 0 to +20°and notches with different depth from 4 to 10 mm. The verification of the model is forthe time being based on two probes: a miniature angle beam probe with 8 x 9 mm²transducer size, 2 MHz frequency and 45° angle of incidence for shear waves. The agreement between the measurements and the theoretical model encourages the use of the clescribed methodology also in other cases, e.g. mode conversion techniques like theLLT-technique as a tandem-replacement approach or for defect characterization and sizing based on a pattern comparison.


R. Boehm, H. Wüstenberg, E. Schulz, A. Erhard, BAM-Berlin


Kurzfassungen der Vorträge und Plakatbeiträge der ZfP Jahrestagung 1996.
Hier der Bericht über die Jahrestagung or Conference Report in English.
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